Swirl flow in combustors has been shown to be advantageous because the high centrifugal forces generated during operation may be utilized to advantageously modify flame turbulence. However, in small scale apparatus, wherein fuel is frequently atomized by high velocity air as a result of the pressure drop across an air swirler, the resulting fuel droplets are rapidly accelerated up to the speed of the air and mixed with the air as a consequence of the extremely high centrifugal forces present. When employed with turbines, at low turbine speeds and at low air temperatures, much of the fuel fails to evaporate and impinges as small droplets on the wall of the combustor. Tough starting of a turbine engine at low rotational speeds is a result.
This difficulty may be alleviated by upsizing the combustor but where the turbine is to be employed in an airborne environment, which is frequently the case and even the cause of low temperature, size and mass problems are compounded.
Moreover, in small scale turbines employing small scale combustors, a low cost apparatus is a commercial necessity. The injectors employed must be downsized and consequently must be simple and inexpensive to form. This in turn frequently means that the size of the fuel droplets generated during atomization by such small injectors increases. The increased droplet size means, of course, greater mass and greater susceptibility to the action of centrifugal force resulting from swirl flow.
Furthermore, even when starting ability is not of primary concern, the premixing of fuel and air as mentioned previously tends to lower stability of the flame and frequently, gaseous phase smoke is produced at fuel rich conditions that are provided to improve flame stability. This in turn results in high flame radiation. The combustor walls necessarily must absorb such radiation and as a consequence, the cooling of the combustor walls becomes more difficult.
The present invention is directed to overcoming one or more of the above problems.